1. Technical Field
The present invention relates to an image forming apparatus, and in particular to an image forming apparatus that forms an image by developing, with toner, electrostatic latent images formed on plural image carriers and transferring the resulting toner images to a transfer medium.
2. Related Art
Conventionally, among image forming apparatus such as laser printers, facsimile machines, and copiers, there has been an image forming apparatus disposed with a photoreceptor drum serving as an image carrier, in which the surface of the photoreceptor drum is uniformly charged and irradiated with a light beam based on image data to form an electrostatic latent image on the photoreceptor drum, the electrostatic latent image is developed using toner charged to a predetermined polarity, and the image developed on the photoreceptor drum is transferred to a transfer medium (paper) using a transfer member such as a transfer roll to form an image.
In order to precisely transfer the charged toner to the paper at this time, it is necessary to cause the transfer member to generate a predetermined transfer electric field and for the transfer electric field to be stabilized by transfer bias control.
Further, in the case of a paper conveying tandem color image forming apparatus disposed with plural photoreceptor drums and transfer members along the conveyance path of the transfer medium, it is necessary to change the voltage applied to each of the transfer members and to change the electric field of each of the transfer members as the paper is conveyed and the toner images are superposed and formed.
In the case of the paper conveying tandem color image forming apparatus, the voltages applied to the transfer members may be gradually raised proceeding downstream along the paper conveyance path.
Incidentally, with respect to the material of the transfer rolls, conventionally transfer rolls or the like in which carbon black is dispersed have been used, but because there are large variations in their resistance values per product, recently ion conductive transfer rolls are being used. Ion conductive transfer rolls have the characteristics that there are small variations in their resistance values between products and their electric resistance drops as their temperature rises.
In a conveyor belt system where a fixer is disposed in the vicinity of the outlet and which has a layout proceeding upward in a substantially vertical direction, it is easy for the upper side transfer rolls, that is the downstream transfer rolls, to be affected by the fixing heat. When ion conductive transfer rolls are used, the temperatures of the upper side transfer rolls rise due to the fixing heat and their electric resistances become smaller. When the transfer electric fields of the transfer rolls are raised from upstream to downstream along the paper conveyance path in this state, the downstream transfer rolls come to have an excessive transfer electric field, which leads to image quality defects.
When the distance between the transfer rolls and the fixer is increased so that the transfer rolls are not affected by the fixing heat, the entire apparatus inevitably becomes larger. And even though an example has been described here where the transfer members comprise transfer rolls, the same is true of film-like members and brush-like members that are ion conductive.